1. Field of the Invention
This invention relates to synthetic hectorite coated films and in particular to a novel system wherein a synthetic hectorite of a particular type is formulated with an air curable hydrophilic epoxy ester binder so as to obtain a coating which can be applied to a wide variety of substrates such as polyolefin packaging films so that the films are enhanced in a wide variety of properties including antistatic properties.
2. Description of Related Art
Synthetic hectorite, commercially available as Laponite.RTM. from Southern Clay Products, Inc., a subsidiary of Laporte, Inc., has been widely used in the art for a wide variety of purposes including its use for imparting antistatic properties.
One field where antistatic properties are desirable is in the use of packaging films, particularly packaging films formed from plastic materials. Films of this type are extremely well known in the art and include a wide variety of polyolefin films, in particular, polypropylene and polyethyiene films as well as other films such as polystyrene films, polyester films, polycarbonate films, polyamide films, polyaramid films, and fluoropolymer films.
There have been two general procedures which have been employed in the art in order to impart antistatic properties to various plastics, the first being the inclusion of various inorganic minerals, including hectorite, into the plastic prior to forming it into a film. The second approach has been to coat plastic films with a coating composition containing various inorganic materials, including synthetic hectorite.
There have been many problems associated with coating films in order to provide a protective layer which has desirable properties and certain of these disadvantages are set forth in U.S. Pat. No. 4,786,588. As is disclosed in said patent, protective layers can have insufficient covering properties and the surface of the layer can become rough and/or the coating can flake off during use.
One approach towards solving the problem is the approach taken in U.S. Pat. No. 4,786,558 in that the inorganic material which is used is modified by treating it with various ions, etc.
A second approach taken in the prior art is disclosed in Japanese publication H3-169540 wherein various plastic films are coated with Laponite.RTM. and various non-epoxy water-soluble binders and adhesion and other desirable properties are alleged to be improved by the inclusion of kaolin. The Japanese publication teaches that without the addition of kaolin, adequate materials are not prepared.
A third approach is disclosed in U.S. Pat. No. 4,868,048 wherein certain fractions, i.e., neighborite is removed from synthetic hectorite before use thereof as a coating with a non-epoxy binder.
It is to be understood that the use of synthetic hectorite as a coating is also disclosed in U.S. Pat. No. 4,173,480 wherein a polymer film base is coated with a synthetic hectorite clay specifically Laponite.RTM. S. The binder is gelatin, starch or carboxy methylcellulose.
It is apparent that there is a continuing need in order to improve the various properties of packaging films, particularly polyolefin films, via a coating technique which would result in films having desirable properties such as non-sticking or blocking, transparency, antistatic properties, and wherein the coating firmly adheres to the film and does not separate or flake off during or after processing.